Chromium based conversion coatings are used to passivate metals such as aluminum, copper, cadmium, zinc, magnesium, tin, silver, iron, and their alloys to reduce and slow corrosion of the metal, or as a finishing coating. Chromium conversion coatings can be applied to everyday items such as tools or hardware to prevent corrosion, and to aerospace and commercial equipment with high requirements for corrosion durability.
Traditionally, chromic acid was used to create conversion coating. However, chromic acid contains high levels of hexavalent chromium. Hexavalent chromium was used to create conversion coatings due to its high oxidation state, resulting in highly effective anti-corrosion coatings. Specifically, hexavalent chromium based conversion coatings prevent oxide formation on the surface of the metal, are conductive, thin and flexible, provide adhesion for other coatings such as adhesives, paints, sealants, and substantially slow corrosion.
However, hexavalent chromium is now known to be a dangerous toxin and a known carcinogen. Chronic inhalation of hexavalent chromium increases risk of lung cancer among other health complications. The presence of hexavalent chromium in drinking water has created substantial health risk as well. For this reason, hexavalent chromium is heavily regulated in both the U.S. and abroad. In 2017, the EU will ban hexavalent chromium for many applications unless an authorization for a specific application or use has been granted.
Corrosion resistance of a conversion coating is a function of the amount of hexavalent chromium on the surface of the coating. Thus, industry has been actively trying to find a substitute for hexavalent chromium based conversion coatings. No alternatives to hexavalent chromium coatings have exhibited as high a corrosion resistance. Specifically, many inventors have tried to use non-toxic trivalent chromium solutions to passivate metals and create corrosion resistant coatings. In these methods, trivalent chromium solutions are used during processing rather than hexavalent chromium solutions. The suggested methods, such as that disclosed in U.S. Pat. No. 5,304,257 to Pearlstein et al. (“Pearlstein”), do not create corrosion coatings that are as effective as the previous hexavalent chromium based coatings.
Pearlstein discloses a method of making a coating which uses an immersion bath of aqueous trivalent chromium to coat an aluminum substrate. After the substrate is removed from the bath, the coating is exposed to an oxidizing solution such that a small portion of the trivalent chromium is converted to hexavalent chromium. However, the method disclosed in Pearlstein contaminates the oxidizing solution and any subsequent rinse waters with hexavalent chromium, creating a chemical waste stream and a chemical exposure hazard.